Moisture-curing adhesives and their preparation belong to the general state of the art and are frequently described in the literature. All isocyanate group-containing prepolymers that are not stored in an absolutely moisture-tight container lose isocyanate groups over time due to reaction with atmospheric moisture. Temperature stress markedly promotes this process. This reaction proceeds rapidly on the surface; diffusion into the interior of e.g. mouldings, foams or layers of adhesives and sealants can take long periods of time. As long as this reaction takes place, the molecular weight or crosslinking density increases and the physical properties change accordingly.
Especially in the adhesives sector, it is desirable for the reaction of the free isocyanate groups of the prepolymer with atmospheric moisture to be as rapid and as complete as possible so that the ultimate use properties are obtained early. Nevertheless, the prepolymer must have a very good storage stability. To accelerate the curing process, external catalysts are often added to the formulations, examples being organic tin compounds (dibutyltin dilaurate) or aminic accelerators (dimorpholinodiethyl ether). However, these catalysts can have an adverse effect on the storage stability under temperature stress of, in particular, prepolymers based on reactive aromatic isocyanates, and on the property profile of the adhesive. There has therefore been no lack of attempts to provide highly reactive isocyanate-containing prepolymers that have a high reactivity to moisture without the addition of external catalysts. Such prepolymers are frequently based on polyetherpolyols or polyesterpolyols containing nitrogen atoms. These products are preferably used in one-component, moisture-curing foam applications.
EP-A 0 002 768 describes, e.g., one-component foams in which isocyanate group-containing prepolymers are present as starting materials. These prepolymers are obtained by reacting a polyol mixture, consisting of 90-100 wt. % of an amino-polyetheralcohol and/or polyesteralcohol, with organic polyisocyanates and other additives, e.g. propellants, in pressurized containers, e.g. aerosol cans. On expansion to atmospheric pressure, e.g., with the aid of a valve, the mixture foams and rapidly cures under the influence of atmospheric moisture to give a dimensionally stable closed-cell polyurethane foam. However, such products are not suitable for the formulation of moisture-curing adhesives.
DE-A 4 417 938 describes isocyanate group-containing prepolymers prepared from organic polyisocyanates and polyols, part of these polyols being based on aromatic aminopolyetherpolyols. In particular, such prepolymers exhibit a reduced tendency to crystallize and are suitable for the production of cellular or compact polyurethane plastics. Such products are not suitable for the preparation of adhesives.
EP 1 471 088 A describes PUR formulations based on polyisocyanates and polyetherpolyols with at least one aromatic and/or aliphatic starter containing NH2 or NHR groups, which, when the polyol component is mixed with the polyisocyanates, immediately or very rapidly produce a good compatibility, so the stirring time can be shortened in comparison with systems of the state of the art. This good compatibility is particularly desirable for the production of PUR casting resins and casting compounds for mouldings which are produced by curing in a mould prepared with release agent.
DE-A 1 922 626 describes a process for the preparation of polyurethane-based one-component systems that are stable on storage and dry rapidly on exposure to atmospheric moisture. The polyurethane isocyanates based on amine-initiated polyethers which can be obtained by this process according to the disclosures and Examples are prepared in substantial amounts of suitable lacquer solvents (60 wt. % of solvent, 40 wt. % of polymer), such as esters, ketones or chlorohydrocarbons, and can be used as binders in one-component coating systems. Suitable solvent-free, low-viscosity one-component adhesives cannot be prepared by this process.
WO 95/10555 describes moisture-curing adhesive compositions consisting of the reaction product of a polyisocyanate and an isocyanate-reactive component containing at least one aliphatic amine-initiated polyol with an ethylene oxide content of at least 1%. Isocyanate-containing prepolymers based on polyethers containing ethylene oxide units have a particularly high affinity for moisture. This can lead to increased blistering while the adhesives are curing. This blistering is disadvantageous for the adhesive bond.
DE-A 10 237 649 describes a one-component polyurethane adhesive containing at least one polyisocyanate prepolymer and at least one aminopolyetherpolyol, the molar ratio of ether groups to amine nitrogen in the aminoetherpolyol being 7 to 30. The proportion of aminopolyetherpolyols in the adhesive according to the invention is very low at only 0.2 to 4.0 wt. %. Even this low proportion, under dimorpholinodiethyl ether catalysis, brings about a halving of the compression time when glueing beechwood. However, this small amount of aminopolyether already has a marked adverse effect on the open assembly time (i.e. processing time).
DE-A 10 304 153 describes a polyurethane prepolymer whose synthetic components are a mixture of polyisocyanates with symmetrical and asymmetrical isocyanate groups, and a mixture of polyethers with functionalities of <2.5 and >2.5, the proportion of polyisocyanates with symmetrical isocyanate groups being greater than the proportion of polyisocyanates with asymmetrical isocyanate groups. The inventive merit of this patent application is said to consist in the fact that the claimed prepolymers are particularly light in colour. This fact is highly dubious since the colour quality of prepolymers depends neither on the isomer composition of polyisocyanates nor on the functionality of polyol components. This fact was taken into account in the granting of the corresponding European patent EP-B 1 490 418, where at least the molecular weights of the polyether components were greatly restricted.
EP-A 1 072 620 describes a solvent-poor adhesive composition consisting of up to 99-999 wt. % of an isocyanate group-containing prepolymer, 0 to 20 wt. % of additives and auxiliary substances and 0.001 to 10 wt. % of at least one morpholine derivative as activator. The compositions described are chosen within very wide limits, so no specific property characteristics of the resulting adhesive compositions are given. Furthermore, a morpholine derivative is necessary, which has a restrictive effect, can have a negative influence on the storage stability and can create compatibility problems.
EP-A 1 072 621 describes fibre-containing adhesive compositions based on the adhesive composition according to EP-A 1 072 620 and 0.1 to 20 wt. % of a filler containing at least one fibre, the difference being that a fibre-containing filler is necessary rather than a morpholine derivative.